1. Field of Invention
The present invention relates in general to continuously variable transmissions, and more particularly, to toroidal type continuously variable transmissions for wheeled motor vehicles.
2. Description of Related Art
As is described in Laid-open Japanese Patent Application (Tokkaihei) 10-331938, a toroidal type continuously variable transmission for use with a wheeled motor vehicle comprises generally an input disc which receives a torque from an engine, an output disc which is coaxially arranged with the input disc and constantly engaged with driven road wheels to drive the same, power rollers which are each interposed between the input and output discs to transmit power therebetween by shearing oil film, and trunnions which rotatably support the power rollers. For controlling the transmission, there are employed an oil pump which is constantly driven by the engine, a forward speed control valve which is used under forward movement of the vehicle, a reverse speed control valve which is used under reverse movement of the vehicle, and a hydraulic servo mechanism which moves each trunnion in a manner to pivot the power roller between a neutral position where a rotation axis of the power roller intersects a common axis of the input and output discs and an offset position where the rotation axis of the power roller is offset toward a shaft axis of the corresponding trunnion.
Upon need of a speed change of the transmission under forward movement of the vehicle, a hydraulic pressure from the oil pump is directed to the hydraulic servo mechanism under control of the forward speed control valve, so that a controlled hydraulic pressure drives servo pistons of the hydraulic servo mechanism for moving the trunnions to pivot each power roller to a desired angled position.
With this, the power rollers are each applied, from the input and output discs, with a component force around the shaft axis of the corresponding trunnion, and thus, each power roller is forced to make a self-inclination around a shaft axis of the trunnion while continuously changing the radius of a circular arc described by the power roller and each of the input and output discs at their contacting portions. With this, a continuously variable speed change is carried out by the transmission.
During the operation, the progress of the speed change is fed back to the hydraulic servo mechanism, so that each trunnion is gradually returned toward the original shaft axis position, and when the speed change ratio actually made in the transmission becomes into coincidence with an instructed ratio, each power roller is returned to the neutral position allowing the transmission to keep the instructed speed change ratio.